Radar level gauge systems are in wide use for measuring filling levels in tanks. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a transmission line probe. The transmission line probe is generally arranged vertically from top to bottom of the tank. The electromagnetic signals are subsequently reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and receipt of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity along the probe of the electromagnetic signals.
In some cases a tank may contain a stratified substance composition with several interfaces between substances at different levels. In such scenarios, it may be necessary to determine the distance between such interfaces to be able to deduce the amount of a certain substance in the tank.
Depending on the substance-dependent and often large attenuation of an electromagnetic transmit signal traveling along a transmission line probe through a substance in a tank, it may, depending on the measurement situation, be practically impossible to determine the levels of all substance interfaces in the tank by means of a conventional guided wave radar level gauge system.
To provide for improved determination of substance interface levels, US 2007/0090992 proposes the use of a transmission line probe with reduced signal coupling to the substance(s) surrounding the transmission line probe. According to US 2007/0090992, this is achieved by means of a so-called Partially External Dielectric (PED) transmission line probe.
Although the PED transmission line probe according to US 2007/0090992 considerably improves the ability to measure interface levels, there are still measurement situations where even the performance provided for through the transmission line probe according to US 2007/0090992 may not be sufficient.
One such measurement situation may be when it is desired to detect a substance, such as sand, having a relatively low dielectric constant, located below another substance, such as water, having a relatively high dielectric constant.
To handle such a situation, WO 2016/025979 suggests to simultaneously measure from the top towards to bottom of the tank, and from the bottom towards the top of the tank. To this end, WO 2016/025979 discloses a sensing element that may include an elongated stainless steel rod and an outer shield, and to enter the tank with a first coaxial cable connected to the steel rod at the bottom of the tank and a second coaxial cable connected to the steel rod at the top of the tank.
Although potentially providing for improved measurement performance in certain measurement situations, the solution according to WO 2016/025979 appears to require a feed-through between the inside and the outside of the tank that can allow passage of several coaxial cables. Such a feed-through appears to be difficult to achieve in practice, in particular when the relevant safety regulations are to be complied with.